Multi section laser diodes are well known in the art and can be switched between different wavelengths. Typically, the diode is calibrated at manufacture to determine the correct controls that should be applied so as to effect the desired output frequencies from the laser.
One of the first known multi-section laser diodes is a three-section tunable distributed Bragg reflector (DBR) laser. Other types of multi-section diode lasers are the sampled grating DBR (SG-DBR), the superstructure sampled DBR (SSG-DBR) and the grating assisted coupler with rear sampled or superstructure grating reflector (GCSR). There are also other laser types such as the External Cavity Laser (ECL) and gas lasers. A review of such lasers is given in Jens Buus, Markus Christian Amann, “Tunable Laser Diodes” Artect House, 1998 and “Widely Tunable Semiconductor Lasers” ECOC'00. Beck Mason.
FIG. 1 is a schematic drawing of a DBR 10. The laser comprises of a Bragg reflector sections 2 with a gain or active section 6 and phase section 4. An anti-reflection coating 9 is sometimes provided on the front and/or rear facets of the chip to avoid facet modes. The Bragg reflector takes the form of Bragg gratings 5. The pitch of the gratings of the Bragg reflector varies slightly to provide a Bragg mode which moves in frequency through varying the current supplied to this sections. The optical path length of the cavity can also be tuned with the phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the DBR and other tunable multi-section diode lasers can be found elsewhere Jens Buus, Markus Christian Amann, “Tunable Laser Diodes” Artect House, 1998.
As detailed above such tunable semiconductor lasers contain sections where current is injected to control the output frequency, mode purity and power characteristics of the device. Various applications in telecommunications/sensor fields require the laser to sweep across a particular wavelength range in as continuous a manner as possible. Moreover many applications require that the range in wavelength that is to be swept to be quite large, up to 80 mm and higher. However a problem with this approach is that certain types of tunable lasers have only a set of narrow ranges of wavelengths over which they can be continuously tuned. These individual continuously tunable regions when put end to end will cover the full sweep range of interest with some overlap.
It is known that lasers can be used to interrogate sensors such as Fibre Bragg Gratings (FBG) which is described in U.S. Pat. No. 5,401,956 and assigned to ‘United Technologies Corporation’. This US patent provides a method of interrogation of a Fibre Bragg Gratings using a tunable laser where the laser is continuously tunable. While it is known that such a laser can be used in DBR or other similar devices a problem with this US patent is that the methodology described is not suitable for discontinuously electronically tunable lasers as it is not reliable and/or accurate for tuning of the laser.
It is desirable therefore to have a scheme by which each of these continuous regions can be seamlessly stitched together to give the appearance of continuous tuning, to overcome the above mentioned problems.